<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
        "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">
<html>
<head>
<title>Commentary on Meta Information Formats</title>
<link rel=stylesheet type='text/css' href='style.css' title='Style'>
<style type="text/css">
<!--
a.ref   { text-decoration: none; font-size: x-small;
          font-weight: normal; vertical-align: super; }
.norm   { font-weight: normal }
-->
</style>
</head>
<body>
<div class='index'>
<b>What sucks:</b>
<br><a href="#EXIF">EXIF / TIFF</a>
<br><a href="#Maker">EXIF Maker notes</a>
<br><a href="#JPEG">JPEG</a>
<br><a href="#IPTC">IPTC IIM</a>
<br><a href="#XMP">XMP</a>
<br><a href="#PDF">PDF</a>
<br><a href="#PNG">PNG</a>
<br><a href="#PICT">PICT</a>
<br><a href="#FlashPix">FlashPix</a>
<br><a href="#DICOM">DICOM</a>
<br><a href="#AVCHD">AVCHD</a>
<br><a href="#ISO">ISO</a>
<br><b>What doesn't:</b>
<br><a href="#CRW">CRW (CIFF)</a>
<br><a href="#QuickTime">QuickTime</a>
<br><a href="#MIE">MIE</a>
</div>

<h2 class='up'>Commentary on Meta Information Formats<br>
<span class='sm'>(or <i>"Why this or that Format Sucks"</i>)</span></h2>

<a name='EXIF'></a>
<h3>EXIF / TIFF</h3>
<p>EXIF uses the TIFF format to store information.  The biggest problem with
this format is that all pointers are absolute (ie. relative to the start of
the file).  This means that the pointers need fixing up if the position of
any information changes.  This is significant because pointers are often
hidden inside proprietary structures which should remain opaque (such as
maker notes).  The result is that some EXIF information is easily corrupted
when editing a file.  This problem is ignored by many software packages.
Much effort was required in the development of ExifTool to avoid the loss of
information such as this.</p>

<p>A significant problem is that an IFD (image file directory) pointer has
the same format code as a long integer, so information in any non-recognized
IFD is lost when the image is rewritten.  There is a seldom-used format
code of 13 that has been proposed to indicate an IFD, but this is not part
of the EXIF/TIFF specification and is not in common usage.  <i class=lt>[Hey!
Olympus has just started using this code for the IFD's in the maker notes of
their new Camera models! - 2007/03/08]</i></p>

<p>Also, it was wrong to specify a count instead of a size for each
directory entry because this makes it impossible to add new format types
without requiring that older readers simply discard data of unknown type
(since the data size isn't known, the old readers don't know how to copy the
data).</p>

<p>All these shortcomings make TIFF a very non-extensible format which by
design will result in the loss of non-standard information.</p>

<p>And we don't learn from our mistakes (OK, maybe some of us can, but
apparently not big corporations like Adobe or Microsoft), because the new
DNG and WDP formats are TIFF-based too.</p>

<p>The EXIF specification also has some significant shortcomings to add to the
TIFF problems, like minimal special character abilities, no language support,
and no way to specify time zone.</p>

<a name='Maker'></a>
<h3>EXIF Maker Notes</h3>
<p>Maker notes are a can of worms.  The EXIF specification unfortunately
made no mention about the information format of the maker notes.  If
properly designed, the specification would have provided a maker note IFD
tag to allow IFD-style maker notes to be used.  Many manufacturers
implemented it this way, but each with a different technique and many with
<a href="idiosyncracies.html">odd quirks</a>. This deficiency combined with
problems in the TIFF (EXIF) format mean that most software will quite
unknowingly scramble your maker notes when an image is rewritten.  To
prevent this, specific knowledge about files written by individual
manufacturers is necessary, and of course the manufacturers are not prone to
divulging this information to the public so it must all be reverse
engineered.</p>

<p>The official TIFF recommendation is to delete unknown information when
rewriting an image<a class=ref href="#ref1">[1]</a>, and this irresponsible
strategy has been adopted by some applications such as Adobe Photoshop,
resulting in a total loss of maker note information. This is very
unfortunate because even corrupted maker notes contain useful information,
and smart utilities (like ExifTool) are often able to recover the
information from damaged maker notes.</p>

<a name='JPEG'></a>
<h3>JPEG</h3>
<p>The JPEG format acts only as a container for meta information, and
defines no meta information format itself.  But it sucks just a as a
container format because it limits the maximum contiguous block size to
64kB.  This causes problems because many cameras store more than 64kB of
additional information in their JPEG images.  Usually, this is due to the
medium-sized JPEG image that many cameras embed for preview purposes. It
would have been nice if there was a standard way to embed this preview in
JPEG images (without the 64kB limitation).  <i class=lt>[Update: In February
2009 CIPA released the MPF format specification which allows storage of
preview images larger than 64kB, but unfortunately this specification
<a href="standards.html#MPF">has problems of its own</a>.]</i></p>

<p>Aside:  The EXIF specification could have easily allowed the information
to span multiple JPEG segments, which would have been smart, but still wouldn't
have solved the problem entirely since it could result in a non-contiguous
preview image (this would be a problem for cameras since they typically want to
use this image for quick review of pictures, and re-mapping the image into a
contiguous memory space would be costly). <i class=lt>[According to the November
2014 XMP specification (part 3, page 13), Adobe allows multi-segment EXIF, and
this is supported by ExifTool 10.97 and later.]</i></p>

<a name='IPTC'></a>
<h3>IPTC IIM</h3>
<p>IPTC IIM earns a few points because it is the easiest to process of the
common meta information formats.  However, the format is overly restrictive
and not very extensible.  Format information and human-readable tag names
are not part of the specification, so it is not possible to meaningfully
interpret unknown values.  Also, there are many interdependencies between
tags that make it very annoying for writing, and the special character
support is ill-conceived.</p>

<a name='XMP'></a>
<h3>XMP</h3>
<p>The XMP format is a good example of a designed-by-committee
specification. It is based on XML, which is much more complicated than
necessary and tries to make everyone happy by providing a multitude of
format styles and features (shorthand format, etc, etc).  This makes it very
difficult to properly implement the complete specification.  Not even the
Adobe software supports all of the format options of their own XMP
specification.  And the format is exceedingly verbose, wasting disk space,
bandwidth and time.</p>

<p>Also, XMP only supports textual information.  Binary information must be
text-encoded if stored in XMP.  This makes it very bulky, slow and
completely unsuitable for storage of binary information.</p>

<p>Mainly because of the complexity of the specification, and despite
Adobe's best efforts, XMP still isn't very well supported by meta
information editing tools.  The best hope for small application developers
is to link against an off-the-shelf XMP or XML library.  Apparently Adobe
realizes this and is now providing an XMP development kit, but this solution
is only useful for those platforms and computer languages that they choose
to support.</p>

<a name='PDF'></a>
<h3>PDF</h3>
<p>...and I thought XMP was too complicated.  PDF is just insane.  Because
of this, it is unlikely that ExifTool will ever support this format for
writing. <i class=lt>[Dec. 18, 2007 Update: I have just added PDF write support
using the incremental update feature of PDF.  It was still a bit of work, but
somewhat simpler (and MUCH faster!) than rewriting the entire PDF.  But there
are disadvantages: The resulting PDF is not "linearized", and there are privacy
and security issues with leaving old metadata in the file.]</i></p>

<a name='PNG'></a>
<h3>PNG</h3>
<p>PNG sucks because meta information can be stored only in text format.
This was a huge mistake, and as a work-around software (eg. ImageMagick)
must encode binary information in ASCII-hex, then compress it to offset the
increase in size, and store it as a compressed text block.  This adds
unnecessary complexity, greatly slows down processing, and makes it much
harder to develop software which supports PNG meta information.
<i class=lt>[July 2017: PNG adopts eXIf chunk to allow EXIF metadata to be
stored in native binary format.  This is useful, but still doesn't allow storage
of arbitrary binary metadata.]</i></p>

<a name='PICT'></a>
<h3>PICT</h3>
<p>The PICT format just plain sucks.  It is one of the worst-designed image
formats in existence, second only to FlashPix.  I don't believe that it was
originally designed as a file format.  Instead it appears to have emerged
from the internal Apple QuickDraw structures.  The files are not designed to
be processed by any software other than the Mac OS.  Intimate knowledge of
the QuickDraw structures is required just to parse the PICT image, even if
the reader doesn't care about the contained information.  Because of this,
PICT images are very fragile and a simple data error or programming bug will
invalidate an entire image.  Possibly for these reasons the PICT format
never gained popularity outside the Apple world.  Also, the concept of meta
information is all but ignored in the PICT design.</p>

<a name='FlashPix'></a>
<h3>FlashPix</h3>
<p>Oh. My. God.  Don't get me started on this one.  Let's just say that basing
an image file format on the File Allocation Table (FAT) structure of a floppy
disk with fixed 512-byte sectors is monumentally idiotic.  Microsoft Word,
Excel and PowerPoint documents also use this insane format.</p>

<a name='DICOM'></a>
<h3>DICOM Medical Images</h3>
<p>The DICOM format is reasonably well designed.  The UID's are cumbersome,
but achieve their purpose.  The biggest bungle in the design is the implicit
VR syntax, which presumably exists for historical reasons to provide
backward compatibility with pre-existing ACR-NEMA images.  The DICOM
specification document is horrible and obviously written by committee, and
was possibly the single largest impediment to implementing support for this
format.</p>

<p>Technical: The files would be easier to parse if the transfer syntax
became effective immediately after the transfer syntax data element instead
of at the end of the meta information group.  Also, the deflated data should
have contained the zlib header, as in PNG and PDF images.  This would make
it easier to read and allow CRC validation of the datastream. -- Can you
tell I wrote this just after adding DICOM support to ExifTool?</p>

<a name='AVCHD'></a>
<h3>AVCHD (.M2TS) <span class='sm norm'><i class=lt>(added 2009-10-29)</i></span></h3>
<p>This format is used in .M2TS video files (and Blu-Ray HD DVD's).  It is
painfully obvious that this was never designed as a storage format.  It uses
MPEG-2 transport stream (M2TS) container, which is a communications protocol
and never should have been used for storage.  The M2TS format is based on a
188-byte packet size which makes no sense for modern filesystems.  And to
make things worse, metadata in AVCHD files is stored in the H.264 video
stream (!!) which uses insane and convoluted data structures such as the
variable-bit-length exponential-Golomb which are painful and inefficient to
parse in software, and as far as I can tell the format for the metadata
stored in these streams is proprietary and undocumented.</p>

<a name='ISO'></a>
<h3>ISO Considered Harmless</h3>
<p>This is not a meta information format, but since many format
specifications are imprisoned by the ISO it deserves mentioning...</p>

<p>In my experience, the International Standards Organization is extremely
counter-productive.  Their goal is to promote standardization, but by
charging too much money for copies of the standards documents they defeat
their own purpose.  The effect is that small companies and individuals
developing software (including open source software such as ExifTool) do not
have access to the official standard.  Instead, software is often based on
old, obsolete documents or drafts which are sometimes publicly available, or
on 3rd party descriptions which are often incomplete.  To sum things up: ISO
sucks.</p>

<hr>
<h2>OK.  So all those formats suck.  What doesn't?</h2>

<a name='CRW'></a>
<h3>Canon CRW Images (CIFF format)</h3>
<p>This format uses a directory structure similar to TIFF, but all offsets
are relative and unknown information can handled properly when rewriting.
Because of this, it is much more extensible than TIFF, but the format is
still limited by 16-bit integer tag ID's among other TIFF weaknesses. It
isn't great, but at least it doesn't suck.</p>

<a name='QuickTime'></a>
<h3>QuickTime Videos</h3>
<p>The Apple QuickTime MOV format is very nice format.  (Which is a breath
of fresh air after the Apple PICT travesty.)  It was a smart move to use
this format for the MP4 specification.  I would only change 2 things about
this format: 1) Use longer tag names instead of the current 4 characters (I
know that 4 characters is convenient because it can be used as an integer in
lookup tables, but it would be nice if the tag names were a bit more
meaningful). 2) Add a format code to the Atom definition so simple unknown
information could be decoded properly.  These changes would allow some
unknown information to be interpreted, which would make it possible for
information added by 3rd parties to be useful.</p>

<p>... but there are some significant problems with the QuickTime specification:</p>

<p>0) The biggest problem with some QuickTime-based files (ie. MOV, MP4,
HEIC) is that atoms may store absolute offsets into the media data, and
these pointers are easily invalidated by editing the file.  This puts a huge
burdon on the editing application to update all necessary pointers, and
makes it much, much harder than it should be to edit these files (it takes
significant effort to just determine which atoms may containing pointers).</p>

<p>1) The metadata organization is a mess because tags may be stored in more
than one location (eg. UserData vs. InfoList), and conflicting metadata
directories may exist, even in the same location (as written by iTunes).</p>

<p>2) The length of 'udta' text strings with language codes is written
incorrectly by many (most?) utilities, including Apple iTunes.  The language
strings are supposed to use a "small integer atom format" where the first
two bytes give the size of the string including the length word and language
codes.  But often the size doesn't include these 4 bytes, which sort of
defeats the purpose of the multiple languages because it is difficult to
parse past the first entry if you don't know the size.</p>

<p>3) Cameras without time zone information are forced to write date/time
tags as local time, even though the specification says these should be UTC.
This leads to an unresolvable inconsistency in the time offset because
one can not distinguish between these two cases.</p>

<a name='MIE'></a>
<h3>My Format (Meta Information Encapsulation, or "MIE")</h3>
<p>Now it is time to put my money where my mouth is:  I have designed a
format which doesn't suck.  It is called "MIE", which stands for Meta
Information Encapsulation.</p>

<p>The MIE format is hierarchical like QuickTime, but it uses meaningful tag
names and specifies a data format for all values.  As well, this format
offers a number of other features:</p>

<ul>
<li>Can be used as a wrapper around any type of file, as a trailer appended
    to other formats, or as a stand-alone meta information format
<li>Extensible
<li>Compact
<li>Meaningful tag ID's
<li>Streamable (and single-pass writing possible)
<li>Relocatable data elements (ie. no fixed offsets)
<li>Relatively simple to implement reader/writer
<li>Supports large data lengths (up to 4GB*4GB)
<li>Localized text feature
<li>Built-in support for numerical units of measure
<li>Multiple documents in a single file
</ul>

<p>The specification for this format can be found
<a href="MIE1.1-20070121.pdf">here</a>, and is implemented in
<a href="http://search.cpan.org/dist/Image-ExifTool/lib/Image/ExifTool/MIE.pm">Image::ExifTool::MIE.pm</a></p>

<p>OK.  Flame away.  I can take it.</p>

<a name="refs"></a>
<h3>References</h3>

<ol>
<li><a name="ref1" href="http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf">http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf</a>, page 26</li>
</ol>

<hr>
<i>Created Nov 15, 2005</i><br>
<i>Last revised Feb 4, 2020</i>
<p class='lf'><a href="index.html">&lt;-- Back to ExifTool home page</a></p>
</body>
</html>
